Context

PR #48588 fixes async server cleanup for Feishu, but the same pattern (sync server.close() without awaiting) likely exists in other channel plugins (Telegram, Slack, Discord, etc.). Each plugin implements cleanup differently, leading to recurring port leak and memory leak bugs.

Proposal

Introduce a PluginResourceManager interface that all channel plugins implement:

• start(accountId): async resource initialization
• stop(accountId): async cleanup with proper await
• cleanup(): full teardown with timeout guards

The gateway can then track all plugin resources centrally and ensure proper cleanup before restart.

Follow-up from #48183 / #48588. Credit to @Artyomkun for the architectural suggestion.